Davis Lab Group: Research

The evolution of limbs from paired fins during the vertebrate invasion of land has inspired the imagination and scientific curiosity of biologists for centuries.  In current models, paired fins are patterned by distinct proximal and distal developmental modules, generating adult skeletal compartments containing either endochondral or dermal elements respectively.  Emphasis on formation of skeletal type led to the hypothesis that fin-folds in fish and autopods (hands/feet) in terrestrial vertebrates are not homologous, patterned by different developmental modules, despite similar distal positions in the appendage.  Recent studies in our lab using two phylogenetically well-positioned vertebrates, the American paddlefish (Polyodon spathula) and the small spotted catshark (Scyliorhinus canicula) have uncovered remarkable similarities in the gene regulatory networks that pattern fin-folds and autopods, supporting the intriguing alternative hypothesis that these structures share a very ancient regulatory homology.

HoxD expression in paddlefish & catshark fin-folds:  implications for paired appendage evolution

The role of Homeobox transcription factors during fin and limb development have been the focus of recent work investigating the evolutionary origin of limb-specific morphologies. We characterized the expression of HoxD genes, as well as the cluster-associated genes Evx2 and LNP, in paddlefish. Our results demonstrate a collinear pattern of nesting in early fin buds that includes HoxD14, a gene previously thought to be isolated from global Hox regulation. We also show that in both paddlefish and catshark late phase HoxD transcripts are present in cells of the fin-fold and co-localize with And1, a component of the dermal skeleton. These new data support an ancestral role for HoxD genes in patterning the fin-folds of jawed vertebrates, and fuel new hypotheses about the evolution of cluster regulation and the potential downstream differentiation outcomes of distinct HoxD-regulated compartments.

 

To read more: 

Tulenko FJ, Augustus, GJ, Massey, JL, Sims, SE, Mazan, S, and MC Davis. (2016)  HoxD expression in the fin-fold compartment of basal gnathostomes and implications for paired appendage evolution. Scientific Reports 6, 22720. doi: 10.1038/srep22720. [PDF]

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Scale bars = 200nm

HoxD expression in the fin-fold compartment of catshark. (a-b) In situ hybridizations in whole-mount and representative cross sections.  (a) In situ expression for the catshark Scyliorhinus canicula Actinodin1 homologue (ScAnd1), shown to be an early molecular marker for cells contributing to the fin-fold in teleosts. Whole-mount ScAnd1 expression in Stage 30 pectoral and pelvic fins is restricted to the distal fin. (a’) cross section of ScAnd1 Stage 30 pectoral fins (and magnification) reveal ScAnd1 positive cells mark the boundaries of the fin-fold with both mesenchymal and ectodermal expression domains. (b) In situ gene expression for S. canicula HoxD12.  Whole-mount HoxD12 expression in Stage 30 pectoral and pelvic fins form distinct proximal and distal domains.  (b’) Cross section of HoxD12 Stage 30 pectoral fins (and magnification) reveal overlap with the distal domain of ScAnd1 (compare to a’), indicating HoxD positive cells in the fin-fold. For all whole mounts: anterior is left, distal is up in pectoral fins, distal is down in pelvic fins.  Dashed lines correlate to plane of section in a’ and b’.

Evolution of fin-fold compartments and the Shh/LIM/Gremlin/Fgf transcriptional network

The evolutionary origin of the autopod involved a loss of the fin-fold and associated dermal skeleton with a concomitant elaboration of the distal endoskeleton to form a wrist and digits. Developmental studies, primarily from teleosts and amniotes, suggest a model for appendage evolution in which a delay in the AER-to-fin-fold conversion fuelled endoskeletal expansion by prolonging the function of AER-mediated regulatory networks. We characterized aspects of paired fin development in paddlefish and catshark to explore aspects of this model in a broader phylogenetic context. Our data demonstrate that in basal gnathostomes, the autopod marker HoxA13 co-localizes with the dermoskeleton component And1 to mark the position of the fin-fold, supporting recent work demonstrating a role for HoxA13 in zebrafish fin ray development. Additionally, we show that in paddlefish, the proximal fin and fin-fold mesenchyme share a common mesodermal origin, and that components of the Shh/LIM/Gremlin/Fgf transcriptional network critical to limb bud outgrowth and patterning are expressed in the fin-fold with a profile similar to that of tetrapods. Together these data draw contrast with hypotheses of AER heterochrony and suggest that limb-specific morphologies arose through evolutionary changes in the differentiation outcome of conserved early distal patterning compartments.

 

To read more: 

Tulenko FJ, Massey JL, Holmquist E, Kigundu G, Thomas S, Smith SME, Mazan S and MC Davis (2017) Fin-fold development in paddlefish and catshark and implications for the evolution of the autopod. Proceedings of the Royal Society B – Biological Sciences 284: 20162780. doi: 10.1098/rspb.2016.2780. [PDF]

Developmental similarities with fin-folds suggest the autopod evolved through changes in the differentiation outcome of conserved early patterning domains.  

 

(a) HoxA13 is expressed in the distal compartment of both  shark (left) and paddlefish (middle) pectoral fins (fin-fold) and in limbs (right; autopod).  

 

(b) Shh/Lim/Gremlin/Fgf network components are expressed in the distal compartment of both fins and limbs, suggesting aspects of AER function are maintained in proximal AF cells.

 

(c) Hypothesis for fin-limb evolution in which the autopod evolved through novel acquisition/expansion of chondrogenic competency in the distal compartment ( * on tree).  In this scenario, dermal skeleton (red) and distal endoskeleton (purple) both reflect distinct differentiation outcomes of the HoxA13 (pink area from a) patterning domain.  Overlap of dermal skeleton and distal endoskeletons seen in Devonian sarcopterygian fossils may reflect this dual differentiation outcome, prior to complete loss of the dermal skeleton in tetrapods.

 

1, catshark; 2, paddlefish; 3, Sauripterus; 4, mouse.